1. Field of the Invention
The present invention relates to an antilock brake control apparatus for use in a vehicle for providing more precise control of the wheel cylinder pressurization and depressurization rate, and providing improved vehicle stability and braking efficiency after recovering from a skid by quickly increasing the wheel cylinder pressure to an appropriate level.
2. Prior art
As shown in FIG. 9, the brake control circuit in a conventional antilock brake control apparatus has a normally-open pressurization valve 3 between the master cylinder 1 and the wheel cylinder 2, a normally-closed depressurization valve 4 and pump 5 in the return channel between the wheel cylinder 2 and the master cylinder 1, and a buffer chamber 6.
When a wheel locking symptom is detected with this antilock brake control apparatus, the pressurization valve 3 is closed and the depressurization valve 4 is opened to reduce the wheel cylinder 2 pressure.
When the locking symptom is overcome, the depressurization valve 4 is again closed. When the wheel speed stabilizes and the wheel speed and estimated vehicle speed are approximately equal, the pressurization valve 3 is opened and closed at a regular pulse to gradually increase the pressure of the wheel cylinder 2 to the master cylinder 1 pressure and thus reduce the pressure difference between the pressure of the master cylinder 1 and wheel cylinder 2. If the brake pressure again increases such that a locking symptom is again detected, the same antilock brake control cycle is repeated.
As shown in FIG. 10, however, the depressurization characteristics of the wheel cylinder pressure in this antilock brake control apparatus have a non-linear curve, and the depressurization time .DELTA.T must be varied according to the pressure of the wheel cylinder to obtain an amount of desired depressurization level, i.e., the depressurization time required to obtain an amount of desired depressurization level .DELTA.P1 increases as the wheel cylinder pressure level decreases. More specifically, depressurization time .DELTA.T2 starting at a lower pressure and a certain period after depressurization begins is longer than the initial depressurization time .DELTA.T1, which starts at a higher wheel cylinder pressure and results in the same decrease in wheel cylinder pressure (an amount of depressurization .DELTA.P1).
Focusing on the proportional relationship between the wheel cylinder pressure and the acceleration or deceleration of the vehicle, methods have been proposed for estimating the wheel cylinder pressure according to the acceleration or deceleration of the vehicle, and obtaining the appropriate depressurization rate by adjusting the depressurization time based on the estimated wheel cylinder pressure.
If the pressure difference between the master cylinder 1 and the wheel cylinder 2 during pressurization is low in this conventional antilock brake control apparatus, however, the rate of increase in the wheel cylinder 2 pressure caused by the pulse-like opening/closing of the pressurization valve 3 will be slow. It will therefore take even longer for the wheel cylinder 2 pressure to become approximately equal to the master cylinder 1 pressure, leading to reduced brake efficiency. On the other hand, if the between difference the master cylinder 1 pressure and wheel cylinder 2 pressure is large, the increase in the wheel cylinder 2 pressure with each opening of the pressurization valve 3 will also be large, making it more likely for a locking symptom to develop again and reducing vehicle stability and control.
In addition, because the wheel cylinder pressure is estimated from the vehicle acceleration/deceleration, there is a delay until the vehicle acceleration/deceleration reaches a level corresponding to the new coefficient of friction .mu. when the friction coefficient .mu. of the road surface changes suddenly during depressurization. High precision control and even obtaining the desired depressurization rate can then be very difficult.